1. Creates a window/dialog with 4 file URL fields (text field + button) and import button.
2. The user selects 4 .c4d files from the /res folder
3. The user hits the import button and the script merges the .c4d files into the current document.

I come from Maya/Python programming, but I'm very new to C4D/Python. I check the C++ and Py docs before asking as well.

Question #01: What do I pass this command?c4d.documents.MergeDocument(doc, name, loadflags[, thread])

What do I pass to "doc"?
I keep my plug-in in the Library->Pref->etc. folder on OSX. How do I get that URL and pass it to "name"?

I've seen a couple scripts that kind of do this but hopefully after I figure out this simple bit I'll be able to hack away at the rest.

Thanks for the response, that worked great and I figured out how to find the /ref folder using the OS module.

I have another questions regarding the UI. How would I go about creating images with text next to them, like the Content Browser? I'd like to write my own custom version of that, but I don't see a way of adding images.

As Bonsak already suggested, you can create your own GeUserArea. Probably the most versatile approach in Python.
A much simpler option would be to use a CustomGUI BitmapButton for the image (turn off button behavior with BITMAPBUTTON_BUTTON) and have the text as static text.
Depends on your needs and the style you want to achieve, what works best for you.